Degradation of Polyalkylene Glycol: Application of FTIR, HPLC, and TGA in Investigating PAG Oil Thermal Stability and Antioxidant Reaction Kinetics

As high performance synthetic oils, polyalkylene glycols (PAGs) have found applications in many industrial machineries either as lubricating oils or as heat transfer fluids despite being susceptible to oxidative degradation in oxygenated environments especially at high temperatures. This study investigates the role of a phenolic antioxidant (AO) in limiting the thermo-oxidative degradation of PAG as a function of the initial AO concentration (3–9 wt %). Kinetics of antioxidant depletion in PAG is also studied over 56 days of accelerated aging at 180 °C. Fourier transform infrared spectroscopy, high performance liquid chromatography (HPLC), and dynamic and isothermal thermogravimetric analysis (TGA) were used to study the changes in the chemistry and thermal stability of the PAG/AO mixtures. Infrared spectroscopy of aged samples revealed the formation of carboxylic acid compounds due to the oxidation of PAG. A HPLC method was developed for quantifying the AO level in PAG during the aging study, allowing for modeling the AO depletion kinetics. TGA revealed significant enhancement in thermal stability of PAG with the addition of AO and an increase of 70 °C in PAG’s onset temperature of decomposition was observed with addition of only 3 wt % AO.